FX has ordered a 15-episode, third season of Robbie Mac's sitcom, It's Always Sunny in Philadelphia, the network announced yesterday. Production begins in March for a summer debut.

Sunny's future had been heavily clouded.

Though its ratings improved 15 percent over the previous season, Sunny averaged only 1.3 million total viewers last season, and didn't bring in big advertising dollars. Still, FX Entertainment chief John Landgraf has faith in the show.

"The bottom line is that I believe in it creatively," he says. "I think it's really funny, a comedic expression of FX's brand. I love working with the guys. We just had to figure out how to produce it more cost-efficiently."

To that end, creator-executive producer McElhenney and his costars agreed to no salary hikes until (or if) FX orders additional seasons. Then the real cheesecake begins.

"John and FX are really hanging their butts out there for us this year, because we're not a big hit," says Robbie Mac, 29, a St. Joseph's Prep grad. "They're investing in the show, which we appreciate. It's not a big issue with us. We're being compensated very well."

Also, Landgraf says he wants to increase the number of episodes produced each season (seven in the first; 10 in the second). A basic-cable show must have at least 80 episodes to reach syndication, he says.

Sunny costars Glenn Howerton, Charlie Day and Kaitlin Olson as politically incorrect Philly pals running a loser bar here. Danny DeVito, who joined the cast last season, will return for all 15 episodes. In addition, he's agreed to stick around for up to three more seasons.

"We didn't even have to ask. He offered," McElhenney says. "Danny had a really good time and we all became good friends. He likes hanging out with us."

Though twice the age of his costars, DeVito is a regular at off-campus cast parties.

He brought his wife, Rhea Perlman, daughter Lucy and several of her friends to a "high school" blast at Day's house. The theme: Dress the way you did in high school.

At the Prep, "I loved it, but I was a real pain," he recalls. "I overcompensated for being small and underdeveloped by being obnoxious and loud. I was a major headache for my parents and the school."

Speaking of major headaches, McElhenney and his buds Howerton and Day wrote virtually every episode the last two seasons. This season, three to four staff writers will be hired.

"It's a big deal," Robbie Mac says. "Hopefully, it will be easier than last year, but you never know. If it doesn't work out, we're prepared to write all 15 episodes. We've got it down to a science."

McElhenney wants to shoot on location in Philly for two weeks this season - double last season's total. The odds "are likely," according to Landgraf.

Hit comedies on basic cable are rare, and Sunny has the potential to become FX's first. Landgraf says he's holding back several other sitcoms in development so Sunny doesn't have to share the spotlight this season.

"People expect our original series to take risks. We tend to have characters on our air who are complicated and dark. The characters on Sunny are worthy to stand on the same stage as antiheroes like Tommy Gavin Rescue Me], Vic Mackey The Shield] and Christian Troy Nip/Tuck]."

On the personal side, Robbie Mac is riding a wave. Literally.

After Sunny wrapped production in July, he learned how to surf during his travels to Central America and Mexico.

"I'm a terrible surfer," he says. "I can paddle out, get up and surf, but I almost get into a fistfight every time because I cut off the other surfers. I don't do it on purpose. How else am I going to learn?"